Human memory T cell differentiation into Th2-like effector cells is dependent on IL-4 and CD28 stimulation and inhibited by TCR ligation

Freshly isolated memory T cells primarily produced IL-2 and small amounts of IL-4 and IFN-gamma after stimulation in vitro. Priming for 5 days in vitro with anti-CD28 monoclonal antibodies (mAb) alone markedly increased production of IL-4. In comparison to fresh cells, the increase in the amount of IL-4 secreted reflected a marked increase in the number of IL-4-producing cells. Stimulation with immobilized anti-CD3 mAb during priming limited subsequent IL-4 production. By contrast, IFN-gamma production from in vitro primed memory T cells was directly correlated to the concentration of priming anti-CD3 mAb. IL-2 production by all restimulated cells was decreased. The differentiation of IL-4-producing cells could be blocked by antibody to IL-4 and enhanced by the addition of recombinant IL-4 as well as antibody to IFN-gamma. Of note, the IL-4-producing effector cells induced from in vitro priming derived from the early CD27pos memory T cell subset, whereas the small CD27neg differentiated memory subset produced IL-4 without in vitro priming. The results indicate that memory T cells can be directed to differentiate into IL-4-producing effector cells by stimulation via CD28 and IL-4, whereas increasing engagement of the TCR limits Th2 memory cell differentiation.     

Evidence for repression of IL-2 gene activation in anergic T cells

The induction of clonal anergy in a T cell inhibits IL-2 secretion because of the development of a proximal signal transduction defect. Fusion of anergic murine T cells to human Jurkat T leukemia cells and formation of heterokaryons failed to result in a complementation of this signaling defect and restoration of murine IL-2 mRNA inducibility. Instead, signal transduction to the human IL-2 gene became disrupted. Heterokaryons formed by the fusion of anergic murine T cells to normal murine T cells also failed to accumulate intracellular IL-2 protein in response to stimulation either with the combination of CD3 and CD28 mAbs or with ionomycin plus a protein kinase C-activating phorbol ester. The results argue against a loss-of-function signaling defect as the sole basis for clonal anergy induction and document the presence of a dominant-acting repressor molecule that inhibits signal transduction to the IL-2 gene within viable anergic T cells.     

CD4-mediated inhibiton of IL-2 production in activated T cells

The role of CD4 in T cell activation has been attributed to its capacity to increase the avidity of interaction with APC and to shuttle associated Lck to the TCR/CD3 activation complex. The results presented in this study demonstrate that ligation of CD4 inhibits ongoing responses of preactivated T cells. Specifically, delayed addition of CD4-specific mAb is shown to inhibit Ag- or mAb-induced responses of both primary T cells and T cell clonal variants. The Ag responses of the latter are independent of the adhesion provided by CD4; thus the observed inhibition is not due to blocking CD4-MHC interactions. Further, analysis of the clonal variants demonstrates that CD4-associated Lck is not essential for the inhibition observed, as anti-CD4 inhibits responses of clonal variants, expressing a form of CD4 unable to associate with Lck (double cysteine-mutated CD4). The inhibition is counteracted by the addition of exogenous IL-2, demonstrating that the block is not due to a lesion in IL-2 utilization, rather its production. It is demonstrated that the delayed addition of anti-CD4 results in a rapid reduction in steady-state levels of IL-2 mRNA in both primary T cells and clonal variants.     

Neuroleptic treatment increases soluble IL-2 receptors and decreases soluble IL-6 receptors in schizophrenia

The cytokines interleukin-2 (IL-2) and interleukin-6 (IL-6) increase during immune activation, they are released from activated astrocytes and microglial cells in the central nervous system (CNS), and they are able to enhance the catecholaminergic neurotransmission. This study focused on the soluble receptors of IL-2 and IL-6 (sIL-2R, sIL-6R) as a part of the regulation system of IL-2 and IL-6. We studied serum levels of sIL-2R in 30 schizophrenic patients not under neuroleptic medication during an acute exacerbation of the disease and reexamined these patients under neuroleptic treatment after clinical improvement. The sIL-6R levels of 39 schizophrenic patients were estimated under the same conditions. The results were compared with the levels of sIL-2R and sIL-6R in 42 healthy controls. No difference was found between the schizophrenic patients before neuroleptic treatment and the healthy controls. During neuroleptic treatment, however, there was a significant increase of sIL-2R levels and a significant decrease of the sIL-6R levels between the pre- and post-conditions. In comparison with healthy controls, the treatment group also showed increased sIL-2R levels and decreased sIL-6R levels. These results suggest that treatment with neuroleptics is associated with increased sIL-2R and decreased sIL-6R. Since sIL-2R bind and inactivate IL-2, whereas sIL-6R form an active complex with IL-6, the increase of sIL-2R and the decrease of sIL-6R together may reflect a functional down regulation of these activating cytokines. This suggests that neuroleptic therapy has a differentiated immunomodulatory effect.     

Mucosal T cells and mast cells share common adhesion receptors

Malondialdehyde (MDA), a lipid peroxidation product, reacts with free amino groups of proteins to give amino-3-iminopropene (AIP) bridges which are immunogenic. Antibodies reacting specifically with MDA-crosslinked proteins preexist in the sera of healthy human subjects, and we hypothesize that they are implicated in processes of senescent cells recognition and elimination. Results obtained by immunofluorescence and flow cytometry show an immunological response against structures arising on cell membranes from MDA reaction with free amino groups of proteins. Such structures might also be implicated in autoimmunity.     

IL-6 rescues resting mouse T cells from apoptosis

It has previously been demonstrated that mature mouse T cells live for many weeks in vivo. In contrast, explanted lymph node or splenic T cells undergo spontaneous death within days, suggesting that survival factors supplied in vivo are not present in normal tissue culture medium. We discovered that IL-6 can rescue resting T cells from apoptosis in vitro. We show that recombinant mouse IL-6 as well as IL-6 in endothelial cell supernatants are sufficient to rescue T cells from death in the absence of additional cytokines. We show that CD4+ T cells express Bcl-2 immediately following isolation from the mouse, but after 24 h in culture Bcl-2 is undetectable. If during this time period the T cells are incubated with rIL-6, Bcl-2 expression is not down-regulated. It is, therefore, possible that IL-6 rescue from death is mediated by maintenance or induction of Bcl-2 expression. Addition of rIL-6 does not by itself induce blastogenesis or proliferation, and therefore, this cytokine appears to be a true survival factor rather than a mitogenic factor for resting T cells. Together, these results support a potential role for IL-6 as one of the factors important for prolonging resting T cell survival in vivo.     

Testosterone induces Ca2+ influx via non-genomic surface receptors in activated T cells

MalK is the ATP-hydrolyzing subunit of the binding protein-dependent ATP-binding-cassette (ABC) transport system for maltose from Salmonella typhimurium. In a recent hypothesis, Glu64 and Glu94 of MalK were proposed as candidates for 'catalytic carboxylate', common to ATP- and GTP-hydrolyzing proteins [Yoshida and Amano (1995) FEBS Lett. 359, 1-5]. Substitution of both residues and, additionally, Glu74 by either glutamine or glycine and valine, respectively, had no deleterious effect on maltose transport. Thus, our data disprove the above notion.     

Prevention of T cell anergy by signaling through the gamma c chain of the IL-2 receptor

When stimulated through their antigen receptor without requisite costimulation, T cells enter a state of antigen-specific unresponsiveness termed anergy. In this study, signaling through the common gamma chain of the interleukin-2 (IL-2), IL-4, and IL-7 receptors in the presence of antigen was found to be sufficient to prevent the induction of anergy. After culture with IL-2, IL-4, or IL-7, Jak3 kinase was tyrosine-phosphorylated, which correlated with the prevention of anergy. Therefore, a signal through the common gamma chain may regulate the decision of T cells to either clonally expand or enter a state of anergy.     

IL-2 rescues antigen-specific T cells from radiation or dexamethasone-induced apoptosis. Correlation with induction of Bcl-2

Most studies of apoptosis on T lymphocytes have examined the effects of various stimuli on immature T cells from the thymus. Previous work has indicated that apoptosis of mature memory T cells may be an important pathophysiologic mechanism in diseases such as AIDS, cancer, and autoimmunity. The effect of IL-2 on apoptosis of T cells is not clear. Therefore, we studied the ability of IL-2 to rescue Ag-specific T cells from apoptosis. We found that IL-2, in a dose-dependent manner, prevented T cells from entering apoptosis induced by gamma-irradiation, mitomycin C, or dexamethasone. This effect was specific for IL-2; IL-1 beta, IL-6, or IFN-gamma could not reproduce it. In contrast to Ag-specific T cells, immature T cells and naive mature peripheral T cells could not be rescued by IL-2 from radiation-induced apoptosis. Apoptosis rescue by IL-2 was associated with the induction of bcl-2 mRNA and protein. This induction could not be attributed to the effects of IL-2 on the cell cycle, as T cells that were prevented from cell cycle progression by irradiation showed a similar induction of bcl-2. Rescued cells retained their Ag-specific proliferative capacity and in vivo functions. These findings demonstrate that the apoptotic death of Ag-specific T cell lines, cells which can be regarded as a model for memory T cells, can be prevented with IL-2. This effect may have important therapeutic implications for patients receiving chemotherapy or radiotherapy, and for patients with AIDS who develop immunodeficiency primarily as a result of loss of Ag-specific memory T cells.     

Viral immune evasion due to persistence of activated T cells without effector function

We examined the regulation of virus-specific CD8 T cell responses during chronic lymphocytic choriomeningitis virus (LCMV) infection of mice. Our study shows that within the same persistently infected host, different mechanisms can operate to silence antiviral T cell responses; CD8 T cells specific to one dominant viral epitope were deleted, whereas CD8 T cells responding to another dominant epitope persisted indefinitely. These virus-specific CD8 T cells expressed activation markers (CD69(hi), CD44(hi), CD62Llo) and proliferated in vivo but were unable to elaborate any antiviral effector functions. This unresponsive phenotype was more pronounced under conditions of CD4 T cell deficiency, highlighting the importance of CD8- CD4 T cell collaboration in controlling persistent infections. Importantly, in the presence of CD4 T cell help, adequate CD8 effector activity was maintained and the chronic viral infection eventually resolved. The persistence of activated virus-specific CD8 T cells without effector function reveals a novel mechanism for silencing antiviral immune responses and also offers new possibilities for enhancing CD8 T cell immunity in chronically infected hosts.     

Human effector memory T cells express CD86: a functional role in naive T cell priming

The glycoprotein CD86 expressed on APCs provides a costimulatory signal necessary for an efficient activation of naive T cells. In contrast, there is controversy about the condition of expression and the function of CD86 on T cells. In this study, we have analyzed the phenotype and the biological activity of CD86+ T cells generated from human PBMC. Results show that CD86 expression on T cells is induced by long term stimulation via CD3 and IL-2R and is down-regulated as the cells become quiescent. The CD86-expressing cells are memory effector T cells: 1) they express CD45RO and high levels of the activation markers CD25, CD54, and HLA-Dr; 2) they selectively express CD30, CD40-ligand, and CD70; and 3) in response to stimulation, most of them produce IFN-gamma before dying by apoptosis. We then analyzed whether CD86 expressed on T cells is functional. Results show that paraformaldehyde-fixed CD86+ T cells enhance the proliferation and production of IFN-gamma by anti-CD3 mAb-stimulated naive T cells and induce proliferation of resting allogenic T cells. All these effects are prevented by neutralizing anti-CD86 mAbs. In contrast, we report no autocrine effect of CD86 in CD86+ T cell activation. In conclusion, these data show that human memory effector T cells express a functional form of CD86 that can costimulate naive T cell responses.     

A model for the interaction of the GM-CSF, IL-3 and IL-5 receptors with their ligands

The high affinity receptors for GM-CSF, IL-3 and IL-5 are heterodimers consisting of a ligand-specific alpha chain and a common beta chain. These proteins are members of a family of proteins known as the "cytokine receptor family" which is characterized by the presence of a 200-residue ligand-binding module. The GM-CSF, IL-3 and IL-5 receptor alpha chains constitute a distinct subgroup and share features not found in other members of the cytokine receptor family, features which we propose to be important for their interaction with the common beta chain and for their binding of the structurally-related ligands. The growth hormone receptor is a well-characterized member of the cytokine receptor family. Based on the structure of the complex between growth hormone and its receptor, we have proposed sites of contact between the GM-CSF, IL-3 and IL-5 receptors and their cognate ligands.     

CD45RA(bright)/CD11a(bright) CD8+ T cells: effector T cells

An important aspect of peripheral T cell development is the differentiation from naive into memory cells. To distinguish naive from memory cells, CD45RA and CD11a are commonly used: CD45RA+ or CD11a(dim) T cells are regarded as naive, while CD45RA- or CD11a(bright) T cells are thought to be of memory type. There is, however, a CD8+ T cell subset which is CD45RA+ and at the same time CD11a(bright). It increases with age and in patients with systemic viral infections, though its functional role in the immune response is unknown. In the present study, we give evidence that this subset is related to memory-like T cells as it produces IFN-gamma and tumor necrosis factor-alpha, contains high levels of perforin, and expresses CD95 in the same way as memory-type CD45RA-/CD11a(bright) CD8+ T cells. Since it contains a high percentage of CD28- and CD57+ cells, is increased in size and granularity, and is transiently expressed following in vitro stimulation of naive CD8+ T cells, we speculate that this subset mainly represents recently activated effector T cells that are able to interact with CD80 and CD86 (B7-1 and B7-2 respectively) negative tissue cells.     

Histamine potently suppresses human IL-12 and stimulates IL-10 production via H2 receptors

IL-12 and IL-10, respectively, stimulate Th1 and Th2 immune responses. The development of some allergic reactions, infections, and tumors are associated with excessive histamine production and a shift toward Th2 responses. Here we address the possibility that this association is causally linked, at least in part, to modulation of IL-12 and IL-10 production by histamine. We report that histamine dose-dependently inhibited the secretion of human IL-12 (p70) and increased the production of IL-10 in LPS-stimulated whole blood cultures. These effects of histamine were antagonized by cimetidine, an H2 receptor antagonist, but not by selective H1 and H3 receptor blockers, and were mimicked by an H2 receptor agonist. The effects of histamine on IL-12 and IL-10 secretion were independent of endogenous secretion of IL-10 or exogenous addition of IL-12, while Ro 20-1724, a phosphodiesterase inhibitor, potentiated the effects of histamine on IL-12 and IL-10 production, implicating cAMP in its actions. Similar modulatory effects of histamine on IL-12 and IL-10 production, which were reversed by the H2 antagonist cimetidine, were observed in PBMC and isolated monocytes stimulated by Staphylococcus aureus Cowan strain 1 and LPS, respectively. Thus, histamine, via stimulation of H2 receptors on peripheral monocytes and subsequent elevation of cAMP, suppresses IL-12 and stimulates IL-10 secretion, changes that may result in a shift of Th1/Th2 balance toward Th2-dominance. This may represent a novel mechanism by which excessive secretion of histamine potentiates Th2-mediated allergic reactions and contributes to the development of certain infections and tumors normally eliminated by Th1-dependent immune mechanisms.     

The role of reactive oxygen species in triggering proliferation and IL-2 secretion in T cells

This paper examines the hypothesis that reactive oxygen species (ROS) play an important role as second messengers in T cell activation. Activation of T cells with phorbol ester in combination with either calcium ionophore, or anti-CD3 antibody results in a large rapid flux of ROS activity. In contrast, co-stimulation with CD28 does not enhance ROS activity. The ROS signal was sensitive to ascorbic acid, desferrioxamine and dimethyl sulfoxide, suggesting that the major active species being generated was the hydroxyl radical, probably by iron-catalyzed decomposition of hydrogen peroxide. The generation of ROS in T cells was regulated by an accessory population within the peripheral blood. An anti-CD2 antibody induced a strong ROS flux, suggesting that the CD2/LFA-3 interaction may be important in this regulation. T cell activation was inhibited by the same panel of anti-oxidants as ROS generation, but much higher concentrations were required for inhibition of proliferation and IL-2 release than those required to block ROS generation. These data imply that ROS are not obligate second messengers for initiation of T cell activation. The results are compatible, however, with a role for activation-dependent T cell ROS generation in modulating the overall T cell response via autocrine and paracrine signalling pathways.